Materials Design for High-Safety Sodium-Ion Battery

Abstract

Sodium-ion batteries(SIB group on linkedin), with their evident superiority in resource abundance and cost, are emerging as promising next-generation energy storage systems for large-scale applications, such as smart grids and low-speed electric vehicles. Accidents related to fires and explosions for batteries are a reminder that safety is prerequisite for energy storage systems, especially when aiming for grid-scale use. In a typical electrochemical secondary battery, the electrical power is stored and released via processes that generate thermal energy, leading to temperature increments in the battery system, which is the main cause for battery thermal abuse. The investigation of the energy generated during the chemical/electrochemical reactions is of paramount importance for battery safety, unfortunately, it has not received the attention it deserves. In this review, the fundamentals of the heat generation, accumulation, and transportation in a battery system are summarized and recent key research on materials design to improve sodium-ion battery safety is highlighted. Several effective materials design concepts are also discussed. This review is designed to arouse the attention of researcher and scholars and inspire further improvements in battery safety.

Heat source:

As shown in Figure 1, the heat generated during the operation of a sodium-ion battery can be divided into three categories: reversible heat Qr, polarization heat Qp and side reaction heat Qs. The reversible heat Qr is usually caused by the reversible entropy change ΔS during the electrochemical reaction. The polarization heat Qp refers to the additional energy consumption caused by ohmic polarization, activation polarization and concentration polarization during the charge and discharge process. of heat. Qs refers to the irreversible heat caused by battery chemical/electrochemical side reactions, including the decomposition of SEI on the negative electrode surface and CEI on the positive electrode surface, reactions between electrolytes and electrode materials, etc. Depending on the electrochemical reaction and the intrinsic properties of the material, Qr may be an endothermic or exothermic process, while Qp and Qs are usually exothermic processes. If the exothermic reaction in the battery gets out of control, a thermal runaway event occurs, which is one of the most catastrophic failure modes of SIBs(SIB group on linkedin).

Thermal runaway process:

As shown in Figure 2, the thermal runaway process consists of three stages: the early stage, the heat accumulation stage and the thermal runaway stage. (1) Early stage. Under normal operating conditions, the battery may have uneven heating rates in various areas due to uneven current density distribution or internal short circuit due to dendrite growth, which may in turn cause local overheating and cause temperature rise. In addition to normal operating conditions, overcharging, exposure to high temperatures, external short circuits, or internal short circuits caused by battery defects can also cause the battery to heat up. Once the battery temperature reaches the threshold for thermal runaway, a self-heating process begins. (2) Heat storage stage. When the temperature reaches the critical temperature, the temperature inside the battery will rise rapidly due to exothermic chemical chain reactions, including decomposition of SEI, reaction between the negative electrode and the electrolyte, separator meltdown, positive electrode decomposition, etc. (3) Thermal runaway stage. When the limiting oxygen index of the system meets the requirements for organic solvent combustion in the electrolyte, thermal runaway breaks out. Eventually, the structure of the sodium-ion battery will be severely damaged, leading to complete battery failure, as shown in the charred and split battery bag in Figure 2.

Important parameters to measure safety: (1) Self-heating temperature Tonset. Tonset refers to the beginning of the self-heating process, the temperature that induces SEI decomposition. The reported tonset of sodium-ion batteries varies greatly from case to case and depends largely on the cell capacity, electrolyte composition and operating conditions. (2) Thermal runaway temperature Te. Te is the turning point temperature between the second and third stages in thermal runaway, and is the highest point at which sodium-ion batteries operate normally. At this critical point, battery temperature increases exponentially. Batteries with higher Te and longer time to reach Te are considered to be safer. (3) Maximum temperature Tmax. Tmax is another parameter closely related to the thermal behavior of the battery. For example, when the battery temperature is higher than the Al foil melting point of 660 oC, the Al current collector melts, causing a short circuit within the battery, thereby releasing more heat. (4) Heating power Q and total calorific value ΔH. Q determines the heating rate of the battery, while ΔH represents the total energy released during thermal runaway. (5) Flammability of electrolyte. It is usually defined by the self-extinguishing time SET or the limiting oxygen index LOI. SET is used to describe how long the ignited electrolyte continues to burn, and LOI is used to quantitatively evaluate the minimum O2 concentration that ensures electrolyte combustion.

Next part about:"Strategies and concepts for high-safety material design" will share in next post.

This article was published on "Advanced Energy Materials" by Chao Yang,?Sen Xin,?Liqiang Mai,?Ya You First published: 18 May 2020

https://doi.org/10.1002/aenm.202000974

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